In the field of electric positioning assemblies, using magnetic position sensors for detecting the position of a movable element of an actuator is well known in the prior art. In most cases, the position detection is performed using a device attached to the actuator, as in application FR288737 which provides using a sensor having a permanent magnet attached to the yoke of the actuator in a configuration enabling the sensor to be insensitive to the magnetic field produced by the actuator. This type of device enables a reliable analogue detection (knowledge of the position of the movable portion relative to the stationary stator portion) but has a major drawback which is its relative overall dimensions which are added to that of the actuator. Although its compact design is often put forward, while ensuring insensitivity to the magnetic field created by the actuator itself, the fact remains that the actuator-sensor assembly is formed of two distinct subassemblies the overall dimensions of which are the sum of the dimensions of the subassemblies. On the other hand, in most actuators, advantage may be taken of a mechanical configuration naturally acting as a magnetic shielding as described in application FR288737. Thus the size of such a solution is not only excessive, but the sensor thus positioned on the top of the actuator requires additional ferromagnetic parts required for shielding the latter so as to minimize the effects of the magnetic fields originating from the actuator on the sensor.
Similarly, the application FR 2791487 is known, which provides for the integration of a magneto-sensitive probe inside the magnetic circuit by creating an additional air gap. The integration of the sensor is thus optimal as regards the overall dimensions, but at the expense of the force or the torque produced by the actuator. As a matter of fact, in the case of actuators with movable magnets, the introduction of an air gap in the stationary or movable ferromagnetic parts entails a modification in the form of the effort produced by the creation of a reluctant (i.e. without current) component which is not constant over the displacement. This not constant force may be a problem in some applications. In addition, the introduction of such additional air gap also reduces the proportional type effort since it induces a greater voltage loss in the magnetic circuit thus modified. Eventually, since the magnetic flux created by the coil also goes through the measurement air gap, the probe signal highly depends on the current and therefore requires knowing current to make the appropriate correction.
In some applications, the performances required by the actuator are such that all the space available is used for its overall dimensions, so that there is no space available to install a sensor at the end or at the periphery thereof. Various solutions then exist to have a built-in position detection, more particularly by measuring the coil inductance when the latter moves on with the position. However, this technique requires, on the one hand, a particularly sensitive and difficult to adjust electronic control, and on the other hand, this cannot be considered for actuators with so-called “proportional” movable magnets, the inductance of which is not or slightly modified according to the position. An additional sensor outside the volume of the actuator must be used then.
U.S. Patent Publication No. 2005/0098153 is also known, which provides for building the magnetic sensor inside the yoke, within a rotary positioning actuator. If the overall dimensions are thus reduced relative to the solutions providing an attached exterior sensor, the non-symmetry of the actuator makes the sensor very sensitive to the field emitted by the actuator.
The Japanese Patent Application JP 2001/214858 discloses an actuator comprising a linear motor and detection means for detecting the position of the drive object. This actuator comprises a detection element which is located outside the axis of symmetry of the actuator, so that the magnetic field insensitivity of the actuator is possible only when using a huge shielding about the detection element.
U.S. Patent Publication No. 2007/0176497 discloses a rotary actuating system comprising a control system and a rotary-linear actuator having a movable plunger and associated coils. The coils may be energized to interact with associated magnets to cause a corresponding displacement of the piston, which may include a rotary and/or linear displacement. The network interface helps the control system of the rotary-linear actuating system to receive control information from the built in network. The control system can control an amplifier for energizing the coils based on the control information.
The patent application EP0076726 discloses a Stirling cycle cryogenic refrigeration unit comprising a reciprocating element accommodated in a body housing containing a working fluid. The mechanical drive means is coupled to said body for reciprocating said body within said housing for selectively expand and compress said working fluid. Non-contact bearing means on said housing can be actuated to center said body in said housing.